In a laser of the kind employing flowing gas as the lasing medium, the gas flows through a discharge cavity where an electric field causes an electric discharge in the gas that produces an emission of light. The amount of light produced is related to the volume of gas that can be made to lase. In moderate and high power gas lasers, the volume of gas that can be made to lase is increased by propelling the gas through the discharge region at high velocity. In high power gas lasers, the gas flow may exceed sonic velocity. To conserve gas, it is conventional to cause the gas to be recirculated in a closed system and to add gas to replace lost gas.
Lasing of the gas causes the gas to become very hot and the gas flowing out of the discharge cavity, therefore, is at a much high temperature than the gas entering the discharge cavity. In a closed system where the gas is recirculated, the hot gas from the discharge cavity flows to a heat exchanger where the gas is cooled to restore the population of gas molecules to levels that are appropriate for again permitting the stimulation of lasing. The cooled gas is then accelerated to increase its velocity and the gas is again directed into the discharge region for the stimulation of lasing.
In gas lasers of the kind in which the gas flows transversely through the discharge region, it is usual to employ a crossflow blower as the device for propelling the gas around its recirculatory path. The crossflow blower is especially suited for use in the transverse flow gas laser because the length of the blower's impeller can be matched to the length of the discharge region.
The crossflow fan appears to have been invented by Mortier in the 1890's and is described in his U.S. Pat. No. 507,445 which was granted in 1893. An improvement on the crossflow fan is disclosed in Datwyler's U.K. Pat. No. 988,712. In that improved fan, the vortex is free to move circumferentially around the fan and the vortex thus is able to adjust its position to prevailing flow conditions. For a discussion on the crossflow fan, see the monograph titled "A Study Of The Cross Flow Fan" by A. M. Porter and E. Markland in the Journal of Mechanical Engineering Science, Vol. 12, No. 6, 1970. That monograph is here incorporated by reference.
U.S Pat. No. 4,099,143 describes a flowing gas laser of the transverse gas flow type having a gas tight cylindrical housing enclosing a crossflow blower, a heat exchanger and means forming a discharge region, together with baffles and vanes for causing the gas to flow in a closed loop. In that arrangement, the crosssflow blower extends longitudinally within the housing along substantially the same length as the discharge region. Consequently, the blower propels the gas transversely through the discharge region along the entire longitudinal extent of that region. In the arrangement disclosed in the patent, the crossflow blower and its baffles are disposed in accordance with conventional practice.
It has been found that the throughput (i.e. the volume of flow) of the crossflow blower is adversely affected where the inlet flow conditions or the outlet flow conditions or both are such as to cause throttling of the flow to occur inside the blower's impeller. Throttling occurs where the center of the vortex generated by the blower is inside the impeller's cage, as described in the Porter and Markland monograph. To assure proper inlet flow conditions, the velocity of the inlet flow must be high enough to prevent the inlet flow from hugging the interior circumference of the impeller's cage. That is, where the velocity of the inlet flow, after once passing through the blades of the impeller, is too low, the flow tends to hug the inside circumference of the cage formed by the blades of the impeller.
It is known from the technical literature that a well behaved crossflow blower generates two vortices and that the main throughput flow of the blower passes between those vortices through a channel bounded by the separation streamlines. The exact location and shape of those vortices are subject to external factors such as ducting and back pressure and in general are not known.